We determined the structure of the a T cell receptor (TCR) b chain complexed with the superantigen staphylococcal enterotoxin B (SEB) to 2.4 [unreadable] resolution. We also determined the structure, to 2.6 [unreadable] resolution, of the complex between the TCR b chain and a mutant of SEB in which valine at position 26 is replaced by tyrosine (SEB V26Y). The crystals belong to space group P21 with cell dimensions a = 71.2 [unreadable], b = 83.6 [unreadable], c = 83.0 [unreadable] for the wild type b-SEB complex and a = 70.9 [unreadable], b = 83.0 [unreadable], c = 82.8 [unreadable] for the mutant b-SEB V26Y complex. There are two complex molecules in the asymmetric unit. X-ray diffraction data up to 2.4 [unreadable] (b-SEB) and 2.6 [unreadable] (b-SEB V26Y) were collected at 100 oK from one flash-cooled crystal for each complex using synchrotron radiation at CHESS beamline F-1 with a Princeton 2K CCD detector. The crystals were soaked in 10% PEG 8000, 24% glycerol, and 0.1 M Tris-HCl, pH 8.5, prior to flash-cooling in liquid nitrogen. Data were integrated and merged using HKL/DENZO/SCALEPACK which gives 34,943 unique reflections with Rmerge= 9.2% for b-SEB and 28,757 unique reflections with Rmerge= 8.1% for b-SEB V26Y. The data sets are 91.5% complete to 2.4 [unreadable] for b-SEB (79.1% from 2.5-2.4 [unreadable]) and 97.0% complete to 2.6 [unreadable] for b-SEB V26Y (90.6% from 2.7-2.6 [unreadable]). The structure of the wild type b-SEB complex was solved by the molecular replacement method with the program AMoRe (Navaza, 1994). The search models consisted of the 14.3.d TCR b chain refined at 1.7 [unreadable] resolution (PDB accession code 1bec) and SEB refined at 1.9 [unreadable] resolution (PDB accession code 1SE4). The structure was refined by iterative cycles of simulated annealing and temperature factor (B) refinement using X-PLOR interspersed with model building into Fo- Fc and 2Fo- Fc electron density maps using TURBO-FROD. The final model contains 7,589 protein atoms and 179 water molecule with Rfree= 0.309 and Rwork= 0.228 in the range 6-2.4 [unreadable]. The r.m.s. deviations from ideal bond lengths and bond angles are 0.006 [unreadable] and 1.79o, respectively. The structure determination of the b-SEB V26Y mutant complex was begun from the partially refined structure of the b-SEB wild type complex. The final model of the b-SEB V26Y complex contains 6,915 protein atoms and 31 water molecule with Rfree= 0.326 and Rwork= 0.229 in the range 6-2.6 [unreadable]. The r.m.s. deviations from ideal bond lengths and bond angles are 0.008 [unreadable] and 1.37o, respectively.